In generators that are used in motor vehicle applications, the demands on the dynamics of the torque are not especially rigorous. In such generators for use in motor vehicles, the torque must not increase too rapidly since otherwise, a powerful generator could in the extreme case stall the internal combustion engine of the motor vehicle. If generators are designed and used as claw-pole generators, they are operated with a diode bridge. In order to increase output, however, attempts have also been made to operate these machines on a pulse inverter. In addition to the requirement for an optimal efficiency for all operating points of the generator, the question of a suitable regulation of the generator over the entire speed and output range also arises.
In current uses of claw-pole generators, they are operated for the most part with attached B6 rectifier bridges. In this instance, electrical machines such as a claw-pole generator offer advantages, among others the fact that they can be produced inexpensively. The advantages of the claw-pole generator principle should be retained as much as possible if, in order to increase the output of the claw-pole generators used, this type of generator is operated with pulse-to-width modulation converters. As a result, the claw-pole generators can also be operated as electric motors. This opens up interesting possibilities for using this type of generator in the starting process of a motor vehicle, transmission synchronizations, and other uses specific to motor vehicles.
In addition to a generator operation of a claw-pole machine, it can also be used in motor operation, where the converter remains unaffected by this change in operating mode. The regulator offers the possibility of affecting the electrical machine, namely the excitation current and the stator current. This permits the achievement not only of a field-weakening, but also of an electric loading in the stator of the electrical machine in the direct-axis direction. Furthermore, by reducing the excitation current of the magnet wheel, a true field-weakening operation can be established. In the claw-pole machine, there is a pronounced magnetic uniaxiality, which depends heavily on the operating point set and therefore influences the capacity of such an electrical machine to be regulated.
EP 0 762 596 A1 has disclosed a motor vehicle generator regulation that produces a direct current, which can be accessed at a direct current output. A three-phase stator winding has three output terminals; a rotor is magnetically coupled to the stator windings in which an electromotive force is produced. Between the direct current output and the three output terminals of the stator winding, a rectifier bridge is connected, which is controlled by means of a regulator in order to control the phase shift between the electromotive force in the stator winding and the phase voltages of the output terminals of the stator winding.
In lieu of diodes in rectifier bridges, triggerable switch elements are used, which can be triggered by means of a separate triggering device.
With the regulating method for electrical machines that is proposed according to the invention, by means of an intentional overmodulation of the pulse-to-width modulation, a reliable transition can be achieved from the sinusoidal regulating operation, into the blocking operation, and on into a second regulation range. Since the regulating strategy for the regulation ranges produced in the claw-pole machine must take into account three different regulation ranges and their requirements, the boundaries of the adjacent regulation ranges must be detected so that a switch from one regulating strategy into another can be executed with no great difficulty. With one and the same regulating structure, PI components included, there can be a transition from the regulation range of a field-oriented regulation to the regulation range in which regulation occurs based on field-weakening, without having to use another regulator. The change of the regulation ranges occurs smoothly and therefore imperceptibly to the outside observer. In contrast to prior regulating methods, which for the most part also operate with sinusoidal voltages and currents, in the regulating method according to the invention, a transition is made to block voltages, which permit an improved voltage utilization.
In addition to the base speed range, a field-weakening operation can also be automatically produced, since the direct-axis current in the electrical machine occurs automatically as a result of machine properties. In prior regulating methods for generators that are used in motor vehicles, tabularly stored interrelationships of engine speed, supply voltage, and the desired output are consulted, which are correlated with the reference currents of the generator to be used. In this case, a voltage regulation over the entire regulation range of the generator is required, which, however, is only possible with voltage reserve when the electrical machine is operated with a converter. Since the converter is limited by the converter ceiling voltage, a reserve must be maintained with regard to this voltage, which reduces the output. This limitation can be overcome through the use of the method proposed according to the invention.